1) The shape of the pinhole. I suppose you could say that the amount of light diffracted is proportional to the perimeter of the pinhole. The smallest perimeter for any given area (e.g. the area of the pinhole itself) is a perfect circle. Anything else adds more 'edge' around which more light can be diffracted. Which results in reducing sharpness. How'd I do?

2) Thickness of the plate material. Let's see. Hmmm.... Imagine light coming into the pinhole at any angle from the axis. Some of that light will "touch" the front edge of the pinhole. Resulting in some diffraction. Some of the light will then "touch" the rear edge of the pinhole. More diffraction. Any thickness greater than zero effectively results in "two" pinholes: one at the front surface of the plate and one at the rear surface. Effectively doubling the diffraction causing edge. Also a thicker plate effectively reduces the pinhole aperture for any light not coming straight from the front.

To remedy this, once you've drilled your pinhole, take a counter sinking drill bit and create a conical shaped hole over your pinhole. If you get it just right, it will result in a near knife edge perimeter for your pinhole. It might make it a bit delicate but no more so than a glass lens.

3) Colour. Not a clue.

I'll go with that. In my own pinhole creations I strive for a clean, round, and very thin pinhole. This has improved my images. I've attempted this with aluminum foil, but ultimately I end up using thin sheet brass. I found that examination under a microscope is necessary to insure there are no entry or exit burrs, and that the pinhole is as clean and round as possible. Black seems to be the color of choice. I have read that blackening the hole with soot from a candle can help absorb light energy that would otherwise be reflected off the inside surfaces of the hole. For my own purposes I've blackened the hole's inside surfaces with black ink. I have included a basic diagram of a pinhole of my own design that has worked quite well so far. If anyone can use this or improve upon it, please feel free to do so.